I like using the diamond abrasive files on mcmaster dot com. Just clamp one end of your knife in a vise, then clamp a piece of straight angle iron on the other end. The angle iron will act as a stop line for where you're filing, the vise will hold the knife and act as a stop line on the opposite end.
Then, you just file in your 90° spine. Afterwards, you'll need to sand the area you filed to your desired finish (I do it by hand).
You'll want to be aware that the portion you have filed and sanded will be finished differently than the rest of the knife. I believe CPK knives typically come with a blasted finish (someone will correct me if I'm wrong). It's an even gray finish over the entire knife. After sanding and filing, the area will be much shinier.
You can leave it that way, it won't hurt anything, but it won't look great. I recommend sanding the entire knife to a satin finish (plenty of youtube how to videos).
If you like the way that looks you can leave it or you can do a quick acid wash to achieve whatever level of dark gray you want.